1. Field of the Invention
This invention relates no a photosensitive material for electrophotography, capable of forming an electrophotographic image. More particularly it relates to an organic photosensitive material or photoconductor for electrophotography.
2. Description of the Prior Art
Organic photoconductors (hereinafter "OPC(s)") have the features such that agents highly sensitive to various wavelengths can be synthesized by molecular design, they are free from environmental pollution and they can enjoy a low cost because of their superior productivity and economical advantages. Thus the OPCs are under energetic research and development. Remarkable improvements have also been made in respect of the durability and sensitivity that have been considered to be problems of the OPCs. Some of them have been put into practical use, and are now prevailing as photosensitive materials for photolithography.
The OPCs are usually used in double-layer structure comprised of a charge-generating layer (hereinafter "CG layer") that absorbs light to generate carriers and a charge transport layer (hereinafter "CT layer") that transports the carriers generated, and it is attempted to to make them more highly sensitive. As agents used in the CG layer (hereinafter "CG agent(s)"), studies are made on various organic agents such as all sorts of perylene compounds, all sorts of phthalocyanine compounds, thiapyrylium compounds, anthanthrone compounds, squarilium compounds, bisazo compounds, trisazo pigments and azuleniun dyes. On the other hand, as compounds used in the CT layer (hereinafter "CT agent(s)"), development has been made on all sorts of hydrazone compounds, oxazole compounds, triphenylmethane compounds and arylamine compounds.
In recent years, there is an increasing demand for the use of OPCs in the near infrared region corresponding with semiconductor laser light (780 to 830 nm), which are used as photosensitive materials for digital recording as in laser printers or the like. Thus, development is energetically made on the OPCs characterized by having a high sensitivity in such a wavelength region. As photosensitive materials for such a region, the OPCs are more advantageous from the viewpoint of sensitivity than inorganic photosensitive materials.
The CG agent and CT agent are applied, together with binder polymers, to a support such as a drum or a belt by relatively simple coating processes to form layers. The binder polymers used for such purpose include polyester resins, polycarbonate resins, acrylic resins and acrylic styrene resins. In the double-layer structure, it is common for the CG layer to be formed in a thickness of several microns and for the CT layer to be formed in a thickness of several ten microns so that a higher sensitivity can be achieved. Here, it is usual for the CG layer to be formed on the support side and for the CT layer to be formed on the surface side on account of their strength, run length, etc. Since what have been put into practical use as the CT agents are only those capable of operating as a result of the transport of positive holes, this double-layer photosensitive material is used according to the negative charge system when it has the layer structure described above.
In this negative charge system, however, there have been the problems that (1) the oxygen in the air turns into ozone, (2) the charge may be imperfect and (3) the photosensitive material tends to be affected by the properties of a drum surface. The ozone is not only harmful to human bodies but also often reacts with the photosensitive material to shorten its lifetime. Instability of charge often brings about a lowering of image quality. The tendency of being greatly affected by the drum surface properties makes it necessary to mirror-finish the drum surface or makes it necessary to provide an undercoat on the drum surface, resulting in an increase in production cost. In addition, in such a double-layer system, there have been the problems that (4) the manufacture requires a complicated process and (5) the layers are questioned about their stability because of their peeling at their interface.
In order to solve such problems, development is energetically being made on OPCs that employ a positive charge system. In order to accomplish the positive charge system, studies have been hitherto made on (a) OPCs of reverse double-layer structure in which the layer structure for the CG layer and CT layer is made reverse to the case of the negative charge system, (b) OPCs of single-layer structure in which a CG agent and a CT agent are dispersed together in a binder polymer, (c) OPCs of single-layer structure in which copper phthalocyanine is dispersed in a polymer, and (d) OPCs of double-layer structure in which an electron-transporting agent that substitutes the conventional hole-transporting agent is used as a CT agent.
In the (a) reverse double-layer structure, like the case of the negative charge system, the problems of the complicated manufacturing process and the peeling at the interface of layers remain unsolved. In addition, since the CG layer, which is essentially required to be made thin, is provided on the surface side of the photosensitive material, a decrease in run length and a deterioration of lifetime characteristics are questioned.
The photosensitive materials aiming at the positive charge system that employs the (b) or (c) single-layer structure have been inferior to conventional double-layer type photosensitive materials of a negative charge system in respect of sensitivity characteristics, charge characteristics (electric charges for charging the photosensitive material can be retained with difficulty) and residual potential (residual potential is large). The reason why they have been inferior in sensitivity is that the generation and transport of electric charges take place at random in one layer. Thus, the problems involved in the single-layer type photosensitive materials are concerned with the sensitivity, the charge characteristics and the residual potential. For this reason, none of single-layer type photosensitive materials have progressed in their practical utilization.
For the (d) method in which an electron-transporting agent is; used as a CT agent, it is still difficult to be put into practical use since no development has been made on good electron-transporting agents.
Compounds known as the electron-transporting agents include 2,4,7-trinitrofluorenone (hereinafter "TFN") and some dephenoquinone derivatives. The TFN has not been actually used because of their problem of cancinogenicity. The diphenoquinone derivatives, on the other hand, are electron-transporting agents recently developed (Yamagushi, Tanaka and Yokoyama, Japan Hard Copy '88 Draft Collections). Studies are made on photosensitive materials of double-layer structure in which this diphenoquinone derivative is dispersed in a polymeric binder and a phthalocyanine pigment, a bisazo pigment, a perylene pigment or the like is used as a charge-generating agent. The photosensitive materials of this type, however, have large residual potential and have been of no practical use.
As discussed above, all the OPCs aiming at the positive charge system have been involved in any problems.
In the meantime, it has been reported that a function-separated type laminated photosensitive material comprised of a CT layer comprising a disperse system of a diphenoquinone derivative and a polymer and a CG layer comprising a phthalocyanine pigment, a bisazo pigment, a perylene pigment or the like can give a superior sensitivity (Yamaguchi, Tanaka and Yokoyama, Japan Hard Copy '88, p. 71). The photosensitive material constituted in this way has so large a residual potential that there are many problems from a practical viewpoint. Moreover, this photosensitive material, which is constituted as a function-separated type (multi-layer structure), has the problems of the complicated manufacturing process and the peeling at the interface of layers.